Determination process for determining if quantum splitting phosphors are obtained and novel compositions

ABSTRACT

Quantum splitting phosphors with host lattices activated by praseodymium +3 where covalency of the Pr +3  -ligand bond is maintained so that the center of gravity of the Pr +3  4f5d band level is located above the  1  S 0  level and the host lattice has a band gap of greater than about 5 eV.

The invention was first conceived or reduced to practice in theperformance of work under a United States Government contract with NIST,having Government Contract Number 0670 NANB 3H1372. The Government mayhave certain rights to the invention.

This application is a Continuation of applicacation Ser. No. 08/624,281filed Mar. 29, 1996 now abandoned.

FIELD OF THE INVENTION

This invention relates to a process for producing quantum splittingphosphors and novel compositions. More specifically, this inventionrelates to quantum splitting in a host lattice by activating the hostwith the praseodymium ion in the +3 state (Pr⁺³). Even morespecifically, this invention relates to rules for obtaining a quantumsplitting phosphor.

BACKGROUND OF THE INVENTION

Quantum splitting is generally described as the conversion of a singleultraviolet photon into two visible photons with the result that thequantum efficiency of luminescence exceeds unity. Quantum splittingmaterials are desirable for use as phosphors in applications requiringlight to be generated. A suitable quantum splitting material can inprinciple produce a significantly brighter light source.

Quantum splitting has been demonstrated previously in fluoride-basematerials. A material comprising 0.1% Pr⁺³ in a matrix of YF₃ has beenshown to generate more than one visible photon for every absorbedultraviolet (UV) photon when excited with 185 nm radiation. The measuredquantum efficiency of this material was 145%±15%, and thus greatlyexceeded unity.

The critical conditions that yield quantum splitting by the Pr⁺³ ionhave been achieved in fluoride-based materials. Recently, quantumsplitting in oxide host lattices activated by Pr⁺³ have been disclosedby applicants in an oxide material having the formula: La_(1-x) Pr_(x)MgB₅ O₁₀, where 0<X≦0.20, U.S. patent application Ser. No. 08/367,634;now U.S. Pat. No. 5,552,082 and in an oxide material having the formula:Sr_(1-x) Pr_(X) Al_(12-x) Mg_(X) O₁₉, where 0<X≦0.20, U.S. patentapplication Ser. No. 08/367,637, now U.S. Pat. No. 5,571 451.

There is a need for a process to produce quantum splitting phosphors andnovel compositions. There is also a need for a process to producequantum splitting in a host lattice by activating the host with thepraseodymium ion in the +3 state (Pr⁺³). There is further a need forrules for obtaining a quantum splitting phosphor activated by Pr⁺³.

SUMMARY OF THE INVENTION

This invention satisfies these needs by providing a process to makequantum splitting phosphors where a host lattice is activated by Pr⁺³comprising the steps of: incorporating the Pr⁺³ in the host lattice sothat the Pr⁺³ energy position of its 4f5d band is located above a ¹ S₀state; positioning the Pr⁺³ 4f5d band and the ¹ S₀ state below the hostlattice conduction band; and choosing a site symmetry of the hostlattice for the Pr⁺³ so that maximum quantum efficiency may be generatedby the phosphor. Examples of host lattices may be oxides, such asaluminates or borates. Other oxide hosts that incorporate praseodymiumin the +3 state in the lattice structure and meet the above-mentionedconditions, are also contemplated as quantum splitting phosphors. Afurther example of a host lattice may be the fluoride hosts that alsomeet the above-mentioned rules for the Pr⁺³ ion. Still a further exampleof a host lattice is a mixture of oxides and halides, such as anoxyhalide host lattice.

Another aspect of this invention is quantum splitting phosphors havinghost lattices activated by the Pr⁺³ ion where covalency of the Pr⁺³-ligand bond is maintained so that the center of gravity (bary center)of the Pr⁺³ 4f5d band level is located above the ¹ S₀ level and the hostlattice has a band gap of greater than about 5 eV.

Still another aspect of this invention includes specific compositions ofquantum splitting phosphors having oxide host lattices activated by thePr⁺³ ion where the Pr⁺³ to oxygen bond distance must exceed about 2.561angstroms (Å) for quantum splitting to occur.

Yet another aspect of this invention includes compositions of quantumsplitting phosphors having fluoride host lattices activated by the Pr⁺³ion where the Pr⁺³ to fluoride bond distance must exceed about 2.28 Å.

Other benefits of this invention may be apprehended by reviewing thedescription and appended claims.

DESCRIPTION OF THE INVENTION

This invention relates generally to new quantum splitting phosphors withPr⁺³ as the activator in a host lattice. The quantum splitting phosphorscan be developed if certain rules are followed. The rules for obtaininga successful quantum splitting phosphor activated by Pr⁺³ are: the Pr⁺³energy band, 4f5d, must be located above the ¹ S₀ state; the band gap ofthe host lattice should be high so that the Pr⁺³ energy band 4f5d andthe ¹ S₀ state are located below the host lattice conduction band; andquantum splitting phosphors will be generated provided that the correctsite symmetry is offered by the host lattice for the Pr⁺³ ion. Forinstance, it has been found that the site symmetry of y⁺³ in YF₃ appearsappropriate for generating high quantum efficiency.

The first requirement for quantum splitting from the Pr⁺³ ionincorporated in a solid is that the 4f5d level is located above the ¹ S₀state. Quantum splitting of the incident high energy photon will notoccur if the energy position of the 4f5d band is below that of the ¹ S₀state. Thus, the location of the 4f5d state depends critically on thestrength of the crystal field acting on the Pr⁺³ ion. A weak crystalfield results in a high energy position of the 4f5d level. Since thestrength of the crystal field decreases with increasing Pr⁺³ ion toanion distance, quantum splitting will be favored in structures whichoffer high coordination number for the Pr⁺³ ion. To demonstrate, quantumsplitting is observed in SrAl₁₂ O₁₉ where the Pr⁺³ ion is coordinated tonine nearest neighbor oxygen ions.

As a result of the crystalline field effect on the energy position ofthe 4f5d level for Pr⁺³ ion, the additional effects of bond covalencyshould also be taken into account. In highly covalent bond surroundings,the center of gravity or bary center of the Pr⁺³ ion 4f5d level islocated at lower energies. Thus, it is necessary to maintain lowcovalency of the Pr⁺³ -ligand bond.

Successful quantum splitting also requires that the Pr⁺³ ion 4f5d bandand the ¹ S₀ state are located below the conduction band of the solidand not within the solid's conduction band. Photoionization of the Pr⁺³ion is possible when its 4f5d level is located within the conductionband of the solid and this may not result in the generation of more thanone photon for every absorbed photon. Therefore, it has been discoveredthat the Pr⁺³ ion must be incorporated in a lattice which has a band gapgreater than about 5 electron volts (eV).

The observation of quantum splitting does not necessarily mean a quantumefficiency exceeding unity. For generating high quantum efficiency it isnecessary that the proper site symmetry be chosen to maximize thecorrect transition probabilities. For instance, it has also beendiscovered that the site symmetry offered by the YF₃ host is ideal forgenerating high quantum efficiency of luminescence.

Other characteristics and advantages of the present invention willbecome more evident from the following examples, which are in no way tobe considered as constituting a limitation on the invention.

EXAMPLES

To illustrate the invention, using the rules given above, the averagePr⁺³ to ligand bond distance in oxide and fluoride hosts has beencalculated. The data in Table 1 shows that for various oxide hosts, thePr⁺³ ion to oxygen bond distance must exceed about 2.561 Å for quantumsplitting to occur.

                  TABLE 1                                                         ______________________________________                                        Average Pr.sup.+3 To Oxygen Bond                                              Distances And Prediction Of                                                   Ouantum Splitting                                                                                  BOND        OUANTUM                                      EXAMPLE   COMPOUND   DISTANCE    SPLITTING                                    ______________________________________                                        1         YPO.sub.4  2.241Å  NO                                           2         YBO.sub.3  2.383Å  NO                                           3         LaPO.sub.4 2.561Å  NO                                           4         LaMgB.sub.5 O.sub.10                                                                     2.611Å  YES                                          5         LaB.sub.3 O.sub.6                                                                        2.616Å  YES                                          6         SrA1.sub.12 O.sub.19                                                                     2.756Å  YES                                          ______________________________________                                    

It is shown by the above data in Table 1 that based on the average Pr⁺³to oxygen ligand bond distance, it can successfully be predicted if thehost lattice will support quantum splitting. It is further noted thatthe above oxide host lattices are for demonstration and do not limit theprocess of this invention to only the above-mentioned oxides. Rather,any host lattice, or mixtures of host lattices, activated by Pr⁺³ ionthat meet the rules of the invention will exhibit quantum splittingphosphors. For instance, it is contemplated that mixtures of oxides andhalides, such as oxyhalides, can serve as host lattices. An example ofan oxyhalide would be LaOCl.

Quantum splitting also requires that the Pr⁺³ ion 4f5d state is locatedat energies below the conduction band of the solid host lattice so thatphotoionization of the Pr⁺³ ion which decreases the luminescenceefficiency is not possible. The Pr⁺³ ion needs to be incorporated into ahost lattice which has a band gap greater than about 5 eV.

Table 2 demonstrates the average Pr⁺³ to halide bond distances and theprediction of quantum splitting for bond distances greater than about2.280Å.

                  TABLE 2                                                         ______________________________________                                        Average Pr.sup.+3 To Halide Bond                                              Distances And Prediction Of                                                   Ouantum Splitting                                                                                  BOND        OUANTUM                                      EXAMPLE   COMPOUND   DISTANCE    SPLITTING                                    ______________________________________                                        7         LiYF.sub.4 2.265Å  NO                                           8         YF.sub.3   2.321Å  YES                                          9         LaF.sub.3  2.364Å  YES                                          10        NaYF.sub.4 2.364Å  YES                                          11        LaCl.sub.3 2.980Å  YES                                          ______________________________________                                    

Variations and modifications will be obvious to one skilled in the artand the claims are intended to cover all modifications and variationsthat fall within the true spirit and scope of the invention.

What is claimed:
 1. A determination process for determining if quantumsplitting phosphors are obtained where a host lattice is activated byPr⁺³, the process comprising the steps of:determining if the Pr⁺³ isincorporated in the host having an effective coordination number for thePr⁺³ ion so that the Pr⁺³ energy position of its 4f5d band is locatedabove a ¹ S₀ state; determining if the Pr⁺³ 4f5d band and the ¹ S₀ stateare positioned below the host lattice conduction band where Pr⁺³ isincorporated in a lattice having a band gap greater than about 5electron volts; and determining if a site symmetry of the host latticefor the Pr⁺³ has been chosen having an effective crystal field strengthand an effective Pr⁺³ to host lattice bond distance; wherein if the Pr⁺³is incorporated in the host having an effective coordination number forthe Pr⁺³ ion so that the Pr⁺³ energy position of its 4f5d band islocated above a ¹ S₀ state, the Pr⁺³ 4f5d band and the ¹ S₀ state arepositioned below the host lattice conduction band, and the site symmetryof the host lattice for the Pr⁺³ has been chosen having an effectivecrystal field strength and an effective Pr⁺³ to host lattice bonddistance a quantum splitting phosphor is obtained.
 2. A processaccording to claim 1 where the host lattice is an oxide host lattice ora halide host lattice or a oxyhalide host lattice.
 3. A processaccording to claim 2 where the oxide host lattice has an average Pr⁺³ tooxygen bond distance greater than about 2.561 Å.
 4. A process accordingto claim 3 where the oxide host lattice having the average Pr⁺³ tooxygen bond distance greater than about 2.561Å is selected from thegroup consisting of SrAl₁₂ O₁₉, LaMgB₅ O₁₀, LaB₃ O₆, and mixturesthereof.
 5. A process according to claim 2 where the halide host latticehas an average Pr⁺³ to halide bond distance greater than about 2.28 Å.6. A process according to claim 1 where the site symmetry of the hostlattice for the Pr⁺³ is chosen so maximum quantum efficiency may begenerated by the phosphor.